Compact discs or CDs are currently manufactured in a relatively complex process in which the information on the CD is first obtained for instance from a digital source. A premastering disc is created in a photo lithographic process which lays down the information in a spiral pattern. From the premastering disc, a master stamping disc is formed in an electroplating process. The master stamping disc is then used to hot stamp thermoplastic discs with the grooves or interstices which carry the information from the premastering disc.
Following the molding of the discs, the discs are "metalized" by placing them, via a vacuum lock, into a vacuum chamber where a thin coat of aluminum is deposited over the physical patterning on the surface of the disc ("substrate") using a sputtering device comprising a magnetron. After metalization the discs are spin coated to cover the metal with a protective coating such as lacquer. This is usually followed by an inspection step.
Compact discs were originally fabricated in a batch process in which individual discs were taken from station-to-station. Present processing requires continuous processing along an efficient flow path to take the discs from start to finish, so that more than 1,000 discs per hour can be manufactured.
The metalization step has caused significant throughput and quality problems in the past. Initially, the metalizer was located off-line (i.e., out of the main, direct line process flow) due to its size and complexity. These units simply could not conveniently be located in the flow path, but rather had to be located off to one side with resultant materials handling problems and complexity.
Metalizers can now be made part of the flow path and are currently available from Leybold in Germany and Balzers in Liechtenstein. The standard metalizers from these companies are similar in design and throughput performance. Both have rotational transport mechanisms which use a dial inside of a vacuum chamber for the transportation of a plurality of substrates (e.g., thermoplastic discs) under a single deposition source which includes a magnetron. Adjoining the vacuum chambers are external rotational transport mechanisms for bringing the substrates (discs) in and out of the vacuum chamber through a single vacuum lock. Internal and external transport mechanisms sequentially carry out the loading and unloading of the substrates.
These metalizers have permitted increased throughput by providing continuous processing. However, the continuous process permits only one title to be run at one time through the molding, metalizing, spin coating, and inspection process. This makes the overall investment for CD manufacturing very high because one metalizing machine must be assigned to each molding system.
It will be appreciated that for compact disc production, orders for compact discs are frequently in the hundreds as opposed to thousands or tens of thousands. Thus, it is very important to be able to either process different titles simultaneously or increase throughput speed to maintain the efficiency of the entire line.
Throughput speed is frequently increased by decreasing the dwell time under the sputtering device. This is accomplished by moving the magnetron closer to the substrate. However, this approach decreases overall quality of the disc and can render high density information discs unusable.
A different approach to increasing throughput speed and also permitting the simultaneous processing of multiple titles, has been developed by Leybold--a double magnetron, double vacuum lock system. Theoretically, this approach should increase disc throughput twofold. However, these machines require the disc to exit to the same vacuum lock it entered resulting in an inordinate amount of indexing complexity which significantly diminishes the theoretical increase.
Leybold's double lock machines have exceedingly large diameter dials, often greater than five feet. The sheer size of the vacuum deposition chamber to accommodate such large dials inhibits the insertion of this machine into the production line, to say nothing of the cost of the relatively large unit. With the use of this device a complicated process flow path is thus, inevitable.
The double magnetron machines also require a double index step in which "odd numbered" discs are processed by one magnetron and "even numbered" discs are processed by the second magnetron. However, with these machines, both discs must pass under both magnetrons. It will be appreciated that with this approach, indexing errors rapidly become significant. When such errors occur during the processing of multiple CD titles, the titles become mixed up such that all of the mixed up discs must be discarded, as there is no way of identifying which disc was associated with which title.
More particularly, when multiple titles are to be processed, the discs of a single title are loaded onto discrete spindles, with an average of six spindles being utilized during a run. These spindles each typically hold as many as 200 CDs. If during the process indexing problems occur, then the entire lot may have to be discarded because the manufacturer must guarantee that the discs loaded on a given spindle come out with the same titles.
Thus while the double magnetron, double vacuum lock system permits processing of multiple titles simultaneously, the potential indexing problems are so severe that such processing is generally not practical There is therefore a necessity for providing an in-line system with efficient metalizing in which indexing problems are reduced to a minimum while at the same time being able to process multiple titles, to accommodate short production runs.